1. Field of the Invention
This invention relates to electrophotographic reproduction apparatus and methods for registering sheets and more particularly to apparatus and methods for control of a stepper motor drive for controlling movement of a receiver sheet into transfer relationship with an image-bearing member that supports an image to be transferred to the receiver sheet.
2. Brief Description of Available Systems
In known electrophotographic copier, printers or duplicators the problem of accurate registration of a receiver sheet with a moving member supporting an image for transfer to the sheet is well known. In this regard, reference is made to U.S. Pat. No. 5,322,273, the contents of which are incorporated herein by reference.
Typically, an electrophotographic latent image is formed on the member and this image is toned and then transferred to a receiver sheet directly or transferred to an intermediate image-bearing member and then to the receiver sheet. In moving of the receiver sheet into transfer relationship with the image-bearing member, it is important to adjust the sheet for skew. Once the skew of the sheet is corrected, it is advanced by rollers driven by stepper motors towards the image-bearing member. During the skew control adjustment, the adjustment is implemented by selectively driving the stepper motor driven rollers, which are controlled independently of movement of the image-bearing member. Typically, movement of the receiver sheet and operations performed thereon by various stations are controlled using one or more encoders. Known registration control systems use a transfer roller with which an encoder wheel is associated. This encoder is used for controlling registration of the sheet. At some point in time after adjustment of the sheet for skew and prior to engagement of the sheet into transfer relationship with the image-bearing member, the control of the stepper motors that provide the drive to the rollers which advance the sheet, is transferred from simulated clock pulses of a microprocessor to the actual clocking pulses generated by the encoder wheel.
A problem with these systems is that in switching control of the stepper motors from synchronization with control signals in the skew correction device to that of the encoder wheel, a stepper motor driving pulse may be lost. This results in sufficient positional difference between receiver sheet and photoconductive belt that accurate registration is not accomplished.
An improved registration apparatus is disclosed in U.S. Pat. No. 5,731,680, the contents of which are incorporated herein by reference. However, even this improved apparatus relies upon a transfer of stepper motor control from simulated clock pulses to the clocking pulses generated by the encoder wheel. The relatively low resolution of the encoder wheels traditionally used in registration systems limits the precision that can be achieved during the transfer of stepper motor control. It is, therefore, an object of the invention to provide improved methods and apparatus for ensuring accurate registration of the receiver sheet and image-bearing member.
In accordance with one aspect of the invention, there is provided an apparatus for advancing a receiver sheet into registered relationship with a moving image-bearing member. The apparatus includes a drive member that engages the receiver. A motor, which is responsive to motor drive pulses, is coupled to the drive member. The apparatus also includes an encoder that generates encoder pulses that correspond with movement of the image-bearing member. A pulse generator is provided to generate motor drive pulses. The pulse generator is connected to the motor for accelerating the receiver sheet to a speed approximately equal to the speed of the image-bearing member.
In accordance with another aspect of the invention, there is provided a method for advancing a sheet into registered relationship with a moving image-bearing member. An encoder is provided that tracks the movement of the image-bearing member. A motor is also provided. The motor is then driven in response to an output of the encoder to accelerate the receiver movement to a speed substantially equal to the speed of the image-bearing member.